I'm sure that most readers have their frame size dialed-in. Our customers are generally experienced cyclist who've owned enough bikes to know what fits. But we do get an e-mail every week or so asking advice on frame size selection. So here it is.
Now, I don't claim to be an expert on fit and find that even those who do are often not. I've been sized for a frame at least four times using various methods by self-proclaimed experts and been assured that I require a 57cm, a 59cm, a 60cm and a 61cm frame. My perfect top tube length is definitely, 58cm, 59cm, or 60cm, depending on the expert. So here is my non-expert advice on picking a VO frame size:
I generally recommend getting a traditionally-sized frame, one that's larger than many race bike oriented shops would recommend. The traditionally sized frame will allow you to get the handlebars to proper height without a super-tall stem. And it will allow a reasonable stem extension that does not put too much of your weight over the front wheel. The frame will handle better, be more comfortable, and you'll look better riding it.
To measure PBH put on your cycling shoes and stand upright with you back to a wall. Spread your feet to about the same width as bike pedals. Now hold a large book square against the wall and raise it firmly against the aforementioned nether region. Have someone measure from the floor to the top of the book. Do this carefully to the closest millimeter. That is you very own PBH. With PBH in hand we can select the frame size based on stand-over height.
Of course this method is not going to work if you plan on using upright bars. In that case go for the longer top tube since upright bars generally sweep back.
Remember that this post is about buying the right sized VO frame and this method will do exactly that. You'll need more information and experience to find your best position on the frame (or to order a custom frame). A good place to start fine tuning would be by reading Bill Boston's site.